While the enter our cells after vaccination, they do not enter the cell nucleus where our DNA is located. In other words, the mRNA enter a part of the cell that is separate from the DNA. And given a lack of the necessary proteins, the mRNA cannot be converted to DNA and integrated into our genome. Moreover, any long-term changes to our DNA can be ruled out by the fact that the mRNA molecules are quickly degraded by normal cellular processes. While certain viruses (e.g. retroviruses) are capable of reverse transcribing their mRNA into DNA for uptake in a host genome, doing so requires proteins with very specific functions, proteins that coronaviruses lack altogether.